[TowerTalk] Analysis of mast slippage in rotor

[Steve Dyer]

I don't see that it address stainless hardware.
Steve W1SRD
> Lubricating threads makes a big difference, and here is a chart that 
> helps
>
> http://www.portlandbolt.com/technicalinformation/bolt-torque-chart.html
>
> Grant KZ1W
>
> On 10/2/2013 2:51 AM, Jim Smith wrote:
>> Hmm.. I suspect that you know more about this stuff than I do.  
>> However, you state, "I did some testing with a spare rotor, a short 
>> piece of mast and a torque wrench. I lubricated the threads to 
>> prevent thread galling."
>>
>> Lubricating the threads drastically changes the torque required to 
>> establish a given amount of stress in the bolt.
>>
>> Fastenal used to have a very good explanation of this topic on their 
>> web site but I don't see it there now.
>>
>> Perhaps others with more understanding of this topic than I would 
>> care to comment.
>>
>> 73, Jim    VE7FO
>>
>> On 2013-10-01 17:00, John Becker wrote:
>>> My mast has slipped about 30 degrees in the rotor after over four years
>>> with no slippage. Prior to noticing this I wasn't aware of any recent
>>> windy days. Before taking corrective action, I'm thinking about why 
>>> this
>>> has happened now and how to most likely prevent a future occurrence.
>>>
>>> The rotor is a HAM-III in a Rohn 25 top section with a Rohn TB-3 thrust
>>> bearing. The antenna is a KT34-XA mounted two feet above the thrust
>>> bearing. This antenna has been up since 1981 and mast slippage has
>>> occurred previously a few times but only when there has been unusually
>>> high wind.
>>>
>>> One of the first things I found is that these rotors apparently use
>>> non-standard size U-bolts. They are 1/4-20 stainless steel with a 2.25"
>>> inside dimension. The only source I found for replacements is Hy-Gain,
>>> now a division of MFJ. I wonder if they are making their own U-bolts? I
>>> was unable to find anyone else selling 2.25" ID U-bolts smaller than
>>> 5/16-18.
>>>
>>> I looked for the correct torque spec for bolt tightening. For 1/4-20
>>> stainless, the Standard Dry Torque spec is 75 inch-pounds or 6.25
>>> foot-pounds. This is for a bolted joint and I wonder if it also applies
>>> to a U-bolt? I did some testing with a spare rotor, a short piece of
>>> mast and a torque wrench. I lubricated the threads to prevent thread
>>> galling.
>>>
>>> 75 inch-pounds is not very tight, definitely less than I would have
>>> tightened them if just going by what feels reasonable to me. I 
>>> gradually
>>> increased the torque to 200 inch-pounds, which is the upper limit of my
>>> smaller torque wrench. I was expecting the U-bolt to fail at less than
>>> 200 inch-pounds but it did not. I left it at 200 inch-pounds for 
>>> several
>>> days to see if there would be a delayed failure but it held. However,
>>> 200 inch-pounds feels too tight to me for a 1/4" bolt.
>>>
>>> Due to the design of the HAM series rotors, the rotor casting contacts
>>> only the center 1.5" of the 2.25" ID U-bolt. This permits progressive
>>> tightening of the U-bolt to cause the shape of the U-bolt to distort,
>>> going from a "U" shape to a rounded "V" shape. It was necessary to
>>> tighten the nuts on both sides of the U-bolt by roughly 1/8" to 
>>> increase
>>> the torque from 75 inch-pounds to 200 inch-pounds.
>>>
>>> I'm wondering if this distortion of the U-bolt also occurs slowly over
>>> time, resulting in a gradual loosening of the U-bolt? This could 
>>> explain
>>> why mast slippage becomes a problem as time progresses.
>>>
>>> Another possibility that comes to mind is that the normal stresses that
>>> occur each time the rotor starts and stops might gradually cause the
>>> nuts to loosen in the absence of rust to hold them in place. I plan to
>>> add stainless steel nylon insert lock nuts on top of the standard nuts
>>> on the U-bolts to prevent this.
>>>
>>> The U-bolt that had been tightened to 200 inch-pounds was distorted to
>>> the point that it was very difficult to get it out of the rotor 
>>> casting.
>>> There were obvious bends in the threaded portion just below the nuts.
>>> This is another indication to me that 200 inch-pounds is too tight, and
>>> I would not have used this U-bolt on my rotor.
>>>
>>> I decided to continue the experiment by straightening the test U-bolt
>>> and tightening it with a larger torque wrench until it failed. However,
>>> I didn't get to the point of using the larger wrench because as I was
>>> re-tightening it, this time it failed at between 150 and 175
>>> inch-pounds. Undoubtedly the operation of straightening it weakened it
>>> further than it already was, and I don't have another spare U-bolt to
>>> sacrifice.
>>>
>>> There have been discussions of mast slippage on this list in the past,
>>> but I don't recall anyone discussing the optimum U-bolt tightening
>>> torque. Possibly I just missed seeing it.
>>>
>>> Suggestions and discussion about how to alleviate this problem would be
>>> appreciated. Thanks!
>>>
>>> 73,
>>>
>>> John, K9MM
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
>>>
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